I have a stock jbj Nano cube 28gallon Cf Quad. It has 105 watt c.f., almost 3.8 watts per gallon. Has anyone tried clams in here? Would they survive close to the top? Could they thrive?

I would say no.Maybe a squamosa but im not even sure on that.

they get way too big

Check this out, and laugh at the naysayers

http://www.nano-reef.com/articles/?article=20

ummm okay

I wouldn't put a clam in a tank with CFs unless you are very experienced with keeping clams and have the ability to move your clam to a tank with better lighting if things go south. Also how old is your tank? You don't want to put a clam in a tank under 6 months old they need a mature system to survive.